Posted by Grant Richardson, 20th September, 2015
According to the Guide for teachers (page 11) at ielts.org:
IELTS recognises both British and American English spelling, grammar and choice of words. It also incorporates a mix of native speaker accents from Australia, Canada, New Zealand, the UK and US in the Listening component.
Despite some regional differences, Australians, Americans, British, Canadians and New Zealanders can more or less easily understand each other’s English (assuming regional slang expressions and technical jargon are not used!). Examiners should accept any of those regional varieties.
Caution: Using past simple instead of present perfect
Americans commonly use past simple (“I saw it already”) where present perfect (“I’ve seen it already”) is traditionally used.
However, I have had an American colleague, after having noticed this variation used in a textbook, inform me that he would not use it himself.
So, you should probably avoid the American variation in this case, unless your examiner has already done it (or already did it?) before you do.